The present invention relates to chemical vapor deposition apparatus.
Certain materials such as compound semiconductors are formed by exposing a surface of a workpiece, most commonly a disc-like wafer, to gases at elevated temperatures so that the gases react and deposit the desired material on the surface of the workpiece. For example, numerous layers of III-V semiconductors such as gallium nitride, indium nitride, gallium arsenide, indium phosphide and gallium antimonide and the like can be deposited onto a substrate to form electronic devices such as diodes and transistors and optoelectronic devices such as light-emitting diodes and semiconductor lasers. II-VI semiconductors can be deposited by similar processes. The properties of the finished device are profoundly influenced by minor variations in properties of the various layers deposited during the process. Therefore, considerable effort has been devoted in the art development of reactors and processing methods which can achieve uniform deposition over a large wafer surface or over the surfaces of numerous smaller wafers held in the reactor.
One form of reactor which has been widely used in the industry is the rotating disc reactor. Such a reactor typically includes a disc-like wafer carrier. The wafer carrier has pockets or other features arranged to hold one or more wafers to be treated. The carrier, with the wafers thereon, is placed into a reaction chamber and held with the wafer-bearing surface of the carrier facing in an upstream direction. The carrier is rotated, typically at rotational velocities of several hundred revolutions per minute, about an axis extending in the upstream to downstream direction. Reactive gases are directed in the downstream direction towards the wafers on the carrier from an injector head positioned at the upstream and the reactor. The wafer carrier is maintained at a desired elevated temperature, most commonly about 350° C. to about 1600° C. during this process. The rotation of the wafer carrier helps to assure that all areas of the exposed wafers are exposed to substantially uniform conditions and that helps to assure uniform deposition of the desired semiconductor material. After the wafers on a particular wafer carrier have been treated, the wafer carrier is removed from the reaction chamber and replaced by a new wafer carrier bearing new wafers and the process is repeated with the new wafer carrier.
Many rotating disc reactor designs incorporate a spindle with a disc-like metallic element, referred to as a “susceptor” permanently mounted on the spindle. The wafer carrier to be treated is disposed on the susceptor and held by the susceptor during the treatment process. Heating elements such as electrical resistance elements disposed downstream of the susceptor heat the susceptor and the wafer carrier during the process. More recently, as disclosed in U.S. Pat. No. 6,685,774, the disclosure of which is incorporated by reference herein, “susceptorless” reactors have been developed. In a susceptorless reactor, the wafer carrier is mounted directly onto the spindle of the reactor when the wafer carrier is placed into the reactor chamber for treatment. The surface of the wafer carrier facing downstream is directly exposed to the heating elements. The susceptorless reactor design provides significantly improved heat transfer from the heating elements of the reactor to the wafer carrier and significantly improved uniformity of heat transfer to all areas of the wafer carrier.
A wafer carrier for a susceptorless reactor must incorporate features which allow the wafer carrier to mechanically engage the spindle when the wafer carrier is placed into the reaction chamber. Such engagement must be provided without damaging the spindle. Moreover, the wafer carrier must be formed from materials which retain substantial strength and rigidity at the elevated temperatures employed and which do not react with the gases employed in the process. Although satisfactory wafer carriers for susceptorless reactors can be formed from materials such as silicon carbide-coated ceramic materials, still further improvement would be desirable.